Some of this is not a training problem, and treating it like one wastes time your dog may not have. If they are distressed when you leave, if they growl or guard their food, their toys or you, if they are frightened on the lead, or if anything about them changes suddenly at any age, contact your vet the same day. A tip from a website, ours included, is not the answer. If it cannot wait, contact your nearest emergency vet now.
With a Dachshund, their back comes first. A yelp when you pick them up, a hunched walk, trembling, refusing the steps, or wobbling on their back legs can be a disc, and fast treatment gives the best chance of recovery. Contact your vet now. A sudden new snap when you touch them can also be pain rather than temper. Bumping into things at night, at any age, or a new pot belly later in life, is worth telling your vet. Bloat (GDV, a twisted stomach) can happen in any dog, though it is most common in large, deep-chested breeds. A swollen, tight belly, retching with nothing coming up, sudden restlessness or drooling is an emergency: contact your vet now.
They'll keep burrowing. If they stop climbing the steps to their blanket, or cry when you lift them, contact your vet now.